Infrared and far infrared spectroscopic studies of the adsorption of water molecules on high-area alkali halide surfaces

Abstract

The adsorption of water on a variety of high-area alkali halide surfaces has been studied by infrared and far infrared spectroscopy. On the majority of alkali halides, of which NaCl is typical, the modified infrared bands from adsorbed water indicate the presence of two species (modes of adsorption) of water to the surface. The first Species (I) involves hydrogen-bonding of both OH bonds to halide ions and gives infrared bands at 3 200–3 450 (v.bd) and 1 660–1 610 cm^–1(bd); this species is obtained first at low coverages. The second Species (II) involves hydrogen-bonding of only one OH group to halide ions and gives infrared bands at 3 665–3 710 (sh), 3 450–3 500 (bd) and 1 600–1 610 cm^–1(sh); it occurs at higher coverage, probably on isolated halide ions. The lithium salts show only Species I at all coverages up to a monolayer. Systematic changes are observed in the wavenumbers of the various infrared bands as a function of the anions or cations present.

Far infrared absorptions are found at 185 and 160 cm^–1 for water adsorbed on CsCl and CsI respectively. These are probably the wavenumbers of the vibrations of the water molecule as a whole against the surface, involving stretching of the hydrogen bonds.